Preliminary experiments performed in this laboratory suggest that there is a recombination system in mammalian mitochondria which preferentially resolves unicircular mtDNA dimers to monomers. This recombination system, or resolvase, as it is termed here, appears to be encoded by nuclear genes. As a working hypothesis, it is envisioned that the resolvase activity is a component of the nucleocytoplasmic regulatory mechanism ensuring that there is an appropriate number of mtDNA molecules for the bioenergetic and metabolic requirements of the cells. This hypothesis makes specific experimental predictions which will be critically tested during this project period to confirm the presence of the resolvase and to obtain additional molecular information on its properties in normal cells and in the mutant LD mouse line which contains only mtDNA dimers. Thus, in one set of experiments, the resolution of mtDNA dimers to monomers will be analyzed in detail in a series of cybrid progeny lines in which parental cells of different nuclear genetic backgrounds will be used. Complementary analyses of the origin and rate of accumulation of dimers in the LD line will also be carried out. The second half of this project is a detailed exploration at the molecular level of whether or not mammalian mitochondria contain a system of general mtDNA recombination and, if so, the role of the dimer resolvase system in producing recombinant molecules. Definitive answers to these questions are now obtainable due to the unique collection of mitochondrial mutants that have been isolated and characterized in this laboratory.